Instrument Flight Rules (IFR) govern how pilots fly when they cannot navigate by visual reference to the ground — in clouds, fog, precipitation, or any other conditions that reduce visibility below Visual Flight Rules (VFR) minimums. IFR flight is conducted under ATC separation and control, using aircraft instruments and navigation aids to maintain course, altitude, and awareness. The regulatory framework for IFR operations in the United States is primarily contained in 14 CFR Part 91, Subpart B, which covers general flight rules including IFR-specific requirements for flight plans, equipment, fuel, alternates, and currency. Understanding these regulations thoroughly is essential for any pilot planning to fly in instrument meteorological conditions (IMC) or simply to maintain an instrument rating.
Section 91.173 states that no person may operate an aircraft in controlled airspace under IFR unless that person has filed an IFR flight plan and received an appropriate ATC clearance. This has several practical implications:
Section 91.205(d) specifies the instruments and equipment required for IFR flight in addition to the standard VFR day and night requirements. The IFR-specific additions are:
The common mnemonic for IFR equipment is "GRABCARD" — Generator, Rate-of-turn indicator, Altimeter (sensitive), Ball (slip-skid), Clock, Attitude indicator, Radio, Direction indicator. Pilots should verify the actual regulatory text rather than relying on mnemonics alone, particularly for aircraft with non-standard avionics or for operations requiring specific navigation equipment.
IFR flight is governed by a system of minimum altitudes published on instrument charts. Understanding these minimums is essential for safe and legal IFR flight:
The MEA is the lowest published altitude for an IFR airway segment that guarantees obstacle clearance and adequate navigation signal reception. Flying at or above the MEA ensures you are clear of terrain and obstructions and can receive the VOR, VORTAC, or other navigation signals needed to stay on course.
The MOCA guarantees obstacle clearance along a route segment but only guarantees navigation signal reception within 22 nautical miles of a VOR. If you are more than 22 nm from the navaid, the MEA must be used. The MOCA is depicted on charts with an asterisk (*) before the altitude.
The MRA is the lowest altitude at which a navigation fix can be identified using the available navigation signals. It is the minimum altitude for determining position at certain intersections where the signal geometry requires a higher altitude for reliable identification.
When the MEA increases ahead of you, an MCA specifies the lowest altitude at which you must cross a particular fix to ensure obstacle clearance on the next segment. Crossing below the MCA can result in flying below the MEA on the outbound segment.
One of the most frequently tested IFR regulations is the alternate airport requirement under §91.169. The rule is commonly remembered as the "1-2-3 rule":
If the forecast for the ±1 hour window meets or exceeds 2,000 feet ceiling AND 3 miles visibility, no alternate is required. If either condition falls below those thresholds, an alternate must be filed.
Alternate airport weather minimums: Not just any airport qualifies as an alternate. The minimum forecast weather at the alternate at your estimated time of arrival must be at least 600-foot ceiling and 2 miles visibility for a precision approach (ILS), or 800-foot ceiling and 2 miles visibility for a non-precision approach. Airports without an instrument approach procedure may serve as an alternate only if the forecast weather allows a descent from the MEA to the airport under basic VFR conditions.
Note on "NA" alternates: Some airports are not authorized as IFR alternates — they are marked "Alternate Minimums: NA" in the instrument approach procedure charts. This may be because they lack weather observation capability or because the approach procedures are not suitable for alternate use.
Section 91.167 sets the minimum fuel requirements for IFR flight. No person may operate a civil aircraft in IFR conditions unless it carries enough fuel to:
If no alternate is required (because the destination meets the 1-2-3 rule thresholds), you still must carry enough fuel to reach the destination plus 45 minutes at normal cruise. The 45-minute reserve is a legal minimum — prudent IFR pilots typically carry significantly more fuel in reserve, particularly when flying to congested airspace where holds and delays are common.
An instrument-rated pilot must maintain IFR currency to act as PIC of an aircraft in IMC or under an IFR flight plan. Under §61.57(c), within the preceding 6 calendar months you must have performed, in actual IMC or under simulated instrument conditions under the supervision of a CFII:
These requirements can be accomplished in an FAA-certificated flight simulator, flight training device, or aviation training device (ATD) as well as in actual aircraft. If you become non-current but remain within the following 6 months (the "grace period"), you may re-establish currency in an aircraft with a safety pilot or in a sim without a formal instrument proficiency check. If more than 12 months have passed since your last currency, you must complete an Instrument Proficiency Check (IPC) with a CFII, who will test you against the ACS standards for the instrument rating.
Instrument approaches are the published procedures that allow IFR pilots to descend from the enroute environment to the airport in IMC. They are divided into two primary categories:
Precision approaches provide both lateral (localizer) and vertical (glideslope) guidance. The most common is the Instrument Landing System (ILS), which can support minimums as low as 200-foot decision height and 1/2-mile visibility (Category I). More advanced Category II and III ILS approaches can support lower minimums, including zero-zero operations at some airports. The RNAV (GPS) LPV (Localizer Performance with Vertical guidance) approach provides equivalent precision guidance using GPS and WAAS and can achieve ILS-equivalent minimums.
Non-precision approaches provide lateral guidance only, without a published glideslope. These include VOR, LOC (localizer-only), NDB (non-directional beacon), and RNAV (GPS) LNAV approaches. Instead of a decision height (DH), these approaches have a Minimum Descent Altitude (MDA) — a floor below which you may not descend until the runway environment is in sight and you are in a position for a normal landing. Non-precision approaches typically have higher minimums than ILS approaches.
When the instrument approach does not align with the landing runway, a circling approach allows the pilot to maneuver visually around the airport to land on the appropriate runway. Circling approach minimums are higher than straight-in minimums and are categorized by aircraft approach speed (Category A through D). Pilots must remain within the protected circling area, which is defined by radius from each runway threshold based on aircraft category.
Under §91.173, you must have an ATC clearance before operating in controlled airspace under instrument meteorological conditions (IMC). In practice, this means filing an IFR flight plan and receiving a clearance before entering controlled airspace when the weather is below VFR minimums.
Under §91.169, if from 1 hour before to 1 hour after your estimated time of arrival (ETA), the destination airport's forecast ceiling will be less than 2,000 feet AGL or visibility less than 3 statute miles, you must file an alternate airport on your IFR flight plan.
Under §61.57(c), within the preceding 6 calendar months you must have performed at least 6 instrument approaches, holding procedures, and intercepting and tracking courses using navigation systems. These can be done in actual IMC, a flight simulator, or a flight training device.
Under §91.205(d), IFR flight requires: all VFR day/night equipment plus a generator or alternator, gyroscopic rate-of-turn indicator, slip-skid indicator, sensitive altimeter adjustable for barometric pressure, a clock with hours/minutes/seconds, DME or RNAV equipment above FL240, and gyroscopic pitch attitude and direction indicators.
Under §91.167, for IFR flight you must carry enough fuel to fly to the destination, then fly to the alternate airport (if one is required), and then fly for 45 additional minutes at normal cruising speed. If no alternate is required, you must still carry enough fuel to reach the destination plus 45 minutes.